Polypyrrolidone fibers and process



United States Patent G 3,318,658 POLYPYRROLIDONE FIBERS AND PROCESS Sidney M. Leahy and Albert C. Tanquary, White Bear Lake, Mmn., assignors to Minnesota Mining and Mannfactoring Company, St. Paul, Minn., a corporation of Delaware No Drawing. Filed Dec. 21, 1962, Ser. No. 246,326

12 Claims. (Cl. 8115.S)

This invention relates to new and improved polypyrrolidone fibers and to a process for preparing the same. More particularly, the invention is concerned with new and improved polypyrrolidone fibers prepared by treating said fibers with formaldehyde.

The term polypyrrolidone fibers as used herein is to be understood as including within its scope polypyrrolidone, fibers, filaments, monofilament yarns, staple yarns, threads, cords, and woven and non-woven fabrics, being comprised of at least about 50% by weight of polypyrrolidone and in a more specific sense fabrics comprised of 90% and more of polypyrrolidone monofilament yarn and/or staple. The process is herein described particularly with reference to fibers comprised substantially entirely of polypyrrolidone, but it is understood that this is intended in an illustrative sense and the invention should not be limited thereby but only insofar as the same may be limited by the appended claims.

The polymerization of pyrrolidone to form polypyrrolidone is described in the art and does not form a part of this invention. Melt extrusion of polypyrrolidone has been employed for formation of filaments which lend themselves admirably to the production of fibers and fabrics having desirable properties of moisture absorption and strength. The fibers are generally stretched to improve the mechanical properties by orientation of the structure. Polypyrrolidone fibers, although generally having excellent properties, possess two disadvantages. They are almost excessively receptive to dyes, so much so that they tend to pick up even traces of dyes from very dilute solutions, thus giving rise to problems in lanudering They also suffer to some extent from fibrillation, that is, the fibers as much and in yarns and fabrics, split off fibrils when subjected to wet abrasion.

Fibrillation is a phenomenon induced in fibrous materials by the application of stress, usually in the form of abrasion, and is characterized by the splitting off from the parent filament or fiber of longitudinal sections of material which are usually referred to as fibrils. The dimensions of the fibrils are small compared to those of the original filament fiber. The splitting off of the fibrils is referred to as fiber breakdown and can readily be observed under the microscope, where the presence of fibrils may be seen. In undyed fabrics, however, the presence of fibrils may not be apparent to casual inspection but it is evidenced by dulling of the finish and can be seen on microscopic examination.

Dyed fibers of melt extruded and oriented polypyrrolidone readily display the eifects of fibrillation, although because of the ready dyeability of these fibers the visual effect of fibrillation is rather less pronounced than in the case of less readily dyed materials. As is known to the art, abrasion which causes fibrillation occurring, for example, at wear points of clothing may result in exposure of deeper dyed or undyed portions of the individual fibers with a resultant shade change which is very undesirable for commercial acceptability.

The more serious aspect of fibrillation is the weakening of the fabric as fibers progressively fibrillate and disintegrate. This problem is only encountered as careful study is made for successful commercialization of the 3,318,658 Patented May 9, 1967 fabrics. The effects of fibrillation become evident on repeated laundering and tumble-drying and substantial absence of fibrillation is thus necessary for good launderabilty of a fabric.

It is an object of this invention to provide a process for the benefication of polypyrrolidone .fibers.

It is a further object of the invention to provide fibers of polypyrrolidone having substantially no fibrillation.

Another object of the invention is to provide a process for the production or manufacture of non-fibrillating polypyrrolidone structures.

Other objects and advantages of the present invention will be apparent from the description thereof hereinbelow.

In accordance with the above and other objects of the invention it has been found that substantially non-fibrillating polypyrrolidone fibers are produced by cross-linking of the polymer molecules composing the fibers with aldehydic cross-linking agents, preferably formaldehyde, in the presence of a weakly acidic methylolation catalyst, e.g. ammonium chloride. Preferably, the fibers treated are previously oriented and heat set.

The useful results obtained by the process of the invention are completely at variance with the prior art disclosures relative to the action of aldehydic agents on polyamides.

According to the disclosure of US. Patent No.

2,7 34,004, formaldehyde reacts with polypyrrolidone, e.g.

dissolved in aqueous glycolic or lactic acids, to produce water-soluble N-methylol derivatives which can be used to finish cotton and nylon fabrics.

It has further been shown in US. Patents Nos. 2,5 16,5 62 and 2,540,726 that when poly'amides are treated with formaldehyde, the dye receptivity of these materials is increased.

So far as is known, however, no method has heretofore been known to the art for improving the fibrillation resistance of polypyrrolidone.

It is surprising to find that when polypyrrolidone fibers are cross-linked with formaldehyde by the process of the invention not only is resistance to fibrillation under wet abrasive conditions markedly improved, but, that the already very strong, almost excessive dye receptivity is not enhanced, but is actually somewhat decreased. This is an extremely useful effect because polypyrrolidone fibers tend to be excessively receptive to certain dyestuffs. Furthermore, high dye receptivity tends to result in rapid exhaustion of dye baths with high consumption of relatively expensive dyestuifs as well as difficulty in controlling the uniformity and degree of dyeing attained.

Fundamentally, and in its broadest aspect, the process of the invention comprises the cross-linking of polypyrrolidone fibers under conditions in which the fibers do not dissolve or become appreciably soluble in the agents used in the process.

In its preferred aspects, the invention comprehends a process in which an oriented, heat-set fiber is treated with an aldehydic agent and the product consists of a highly shrink-resistant, fibrillation-resistant fiber which has greatly improved launderability.

Hot tempering, sometimes also called heat setting, may be performed using conventional machinery for the purpose. placed under a tension of at least 0.05 gram per denier and are then subjected to hot tempering conditions.

It will be seen that while tension on fibers as monofilaments, yarns, cords, threads and the like can only be maintained longitudinally, when woven or non-woven fabrics of polypyrrolidone are to be treated by the process of the invention, tension is desirably maintained both longitudinally and laterally of the fabric. It will be evident to those skilled in teh art that diiferent tensions may Fibers which have been previously oriented are be maintained in different directions and that fabrics having, for instance, a warp of polypyrrolidone fibers and a woof of cotton fibers will be handled accordingly.

The application of tension to polypyrrolidone fibers is very easy because hot tempering conditions tend to cause shrinkage of the fibers. The fibers need only to be prevented from shrinking by suitable constraining means. Fibers and filaments may be wound on reels of suitable size and shape, preferably with a foraminous core so that penetration of reactants and convection heating can reach the fibers from both outsideand inside, i.e. maintaining static tension. When a fabric is to be treated, pins in the reel may engage the edges of the fabric to effect lateral tension in the warp. When a non-woven fabric is being treated it may additionally be desirable to provide a compression jacket to minimize any tendency for the fabric to tear itself apart by slipping of the fibers from mutual engagement In addition to the above static methods for maintaining tension, conventional dynamic methods, i.e. continuous processes, are also suitable. Wet hot tempering with steam or hot water is effected at temperatures in the range of about 105 to about 110 C. for about 30 to about 100 minutes, the longer dwell times being employed at the lower temperatures. This wet process is preferred because there is less chance for damaging the fibers, e.g. tendering, at the temperatures used and because slight variations in dwell time have less effect on the hot tempering process.

Dry hot tempering, where the atmosphere used is relatively free from water (e.g. up to 0.2 atmosphere partial pressure), is carried out at temperatures of about 190 to about 235 C. for a time sufficient so that the entire fiber cross-section reaches the desired temperature. When heat is provided to reels of fiber from a hot gas, such as air, which has a low heat capacity, dwell times may be from about minutes at 200 C. down to 3 minutes at 230 C. In a continuous process where the fibers or filaments are heated in a layer only about 1 mil thick by a heated metallic, ceramic or other solid surface, dwell times are much shorter, being from about 5 seconds at 190 C. to about 0.3 second at 235 C. Such short times are obviously advantageous for continuous dynamic hot tempering processes.

Regardless of the method or machinery used and the temperature or moisture condition, the essential feature of the hot-tempering step is to dimensionally stabilize the fiber in oriented condition. Hot tempered fiber is fiber which has been oriented to an appreciable degree and then heated to a temperature sufficient to substantially prevent dimensional change when heated to the boiling point of water.

The second step of the process of the invention consists in cross-linking the polymer molecules of the oriented fibers with formaldehyde in the presence of a weakly acidic methylolation catalyst. For convenience herein, the term formaldehyde is used to encompass aldehydic agents such as formaldehyde and sources thereof, e.g. paraformaldehyde, methylol urea, dimethylol ethylene urea, dimethyl formal, saligenin, formalin, hexamethylene tetramine, and the like.

This may be viewed as occurring in two stages, first, a reaction to partially methylolate the polypyrrolidone molecules, to an extent sufficient to fix the formaldehyde but which does not solubilize the polypyrrolidone and second, elimination of water from the polypyrrolidone containing the fixed formaldehyde to form crosslinks.

In general, the hot-tempered polypyrrolidone fiber (preferably, but not necessarily, still under a certain amount of tension) is most conveniently impregnated with the methylolation catalyst and the aldehydic agent in water solution. Under such conditions, saturation of the polypyrrolidone article effects an uptake of about 4 to of the weight thereof of the saturating solution.

The formaldehyde may be introduced either concomitantly with the aqueous catalyst or subsequently thereto. In the former case, after drying to remove water at a slightly elevated temperature, which possibly methylolates the polypyrrolidone as noted above and thus fixes the formaldehyde, the fiber is heated at a temperature in the range of about 50 to about 150 C. for from about 1 to 60 minutes. Higher temperatures are used with shorter times and vice versa. At the higher temperatures there may be some risk of injuring the fiber, i.e. tendering, unless quite short times of treatment are employed and such temperatures are better utilized in conjunction with continuous operation rather than batch processes.

When the formaldehyde is used separately from the methylolation catalyst the fiber containing the catalyst if first dried and is than exposed to gaseous or vaporous formaldehyde at a temperature of about 120 to about 150 C. for from about 2 to about 30 minutes. It will be evident that the amount of formaldehyde available for reaction will influence the time of reaction in this and other instances as will the bulk of the fibers being treated.

It is preferred that sufficient formaldehyde be available so that from about 0.3 to about 5 percent of formaldehyde based on the weight of the polypyrrolidone is combined in the fibers. However, from 0.1 to about 10 percent by weight of formaldehyde can be reacted with the polymer in the fiber to produce fibers with improved properties. Polypyrrolidone fibers containing from about 0.3 to about 5 percent by weight of formaldehyde bound therein are preferred fibers of the invention. It is believed that when combined in the fiber, the elements of Water are lost from the formaldehyde residues leaving methylene groups as cross-links between polymer chains, but of course this theory is not to be considered as conelusive.

In another aspect, the gel content of the polypyrrolidone after crosslinking is a measure of the desired and essential extent of crosslinking. The gel content of the formaldehyde-treated, crosslinked fiber is determined as follows: One part by weight of treated fiber is added to 50 parts of trifiuoromethanol and the mixture is stirred at about 25 C. for 8 hours, and then is poured into a tared glass suction filter funnel with fritted glass disk. The trifluoroethanol and dissolved polymer is removed by suction, and the remaining insoluble material is Washed with 10 parts of fresh trifluoroethanol. The solvent is again removed by suction and the remaining material is dried in the funnel for 16 hours in a vacuum oven at 120 C., cooled and weighed. The fraction of the original sample which remains, expressed as a percentage, is the gel content.

Treated fiber having a gel content above about has improved properties, with reduced fibrillation. Preferably, the gel content is over Generally speaking, it is found that fibers having or higher gel content are practically non-fibrillating.

Broadly speaking methylolation catalysts which are useful for the purposes of the invention are those reagents or combination of reagents which produce aqueous solutions of pH values ranging from pH 1 to pH 5. Specific examples of such agents include dilute aqueous solutions of strong acids, water-soluble weak acids and salts of acids with weaker bases. Such substances include 0.01 N aqueous hydrochloric acid, ammonium chloride, zinc chloride, sodium bisulfite, ammonium sulfate, oxalic acid and very dilute formic acid. Glycolic and lactic acid are also suitable as catalysts but have the disadvantage of plasticizing the fibers and thus degrading their mechanical properties.

A sufficient amount of catalyst is used to ensure take-up of about 0.1 to 10 percent by weight of formaldehyde; or, alternatively, to provide a gel content in the treated fiber above about 80%, when an excess of formaldehyde is used. When smaller amounts of catalysts are used, larger periods of exposure to formaldehyde may be necessary. When dilute solutions of strong acids are used, it should be noted that the saturated fiber should not be completely dried because the concentration of acid will become excessively great.

Because the acidic catalyst may cause tendering of the fibers, it is desirable that the heating befor periods no longer than the approximate maxima stated hereinabove, where residual fiber strength is a material factor. While not forming an essential part of the process of the invention, it is also desirable to remove the acidic materials from the fibers after cross linking has been effected. Removal of the catalysts is readily effected by washing the fiber with water, for example for a dwell time of about seconds in 50 C. water for open goods. Washing may be delayed until wet finishing of the goods but should be done before hot tentering or calendering. Thereafter, the fiber is dried.

As stated hereinabove, polypyrrolidone fibers treated by the benefication process of the invention, possess lowered dye receptivity and strongly enhanced resistance to fibrillation.

The resistance of the fibers to fibrillation is determined by a useful and reproducible test method which is a variation of the commercial accelerotor abrasion test. To carry out the test, bundles of fibers are mounted on the periphery of a 4 /2 inch diameter disc so that they extend radially for 1 inch. (Eight such bundles can be accommodated at one time without interference one with the other.) The disc is rotated at 1000 r.p.m. for 3 minutes with the bundles of fibers contacting the internal wall of a 5% inch diameter cylinder having 2 small corrugations per inch of periphery on the inner surface each about inch high and with a pool of 35 ml. of water at about 25 C. in the bottom of the cylinder to keep the fibers thoroughly wet. This test corresponds approximately to the effect to be expected from 20 cycles of washing in a household washing machine each followed by tumble drying. This treatment rapidly produces fibrillation of susceptible fibers, the extent of which is designated by numerical scores. To aid in resolving differences a microscope (IOU-200x) is used to examine the ends of the bundles. A score of 6 or higher up to 10 indicates progressively more felting of the fibril ends in a bundle, 10 indicating a state in which the fibers are completely frayed so that each appears pilose. A "score of 6 indicates marked improvement in fibrillation, while a score of 0 indicates no fibrillation. The presence of a moderate number of fibrils, i.e. a score of 3-4 is considered to represent an improvement over unreacted fibers which is practically speaking very useful.

The following examples will serve to show more specifically the best mode presently contemplated of practicing the invention, as well as certain comparable examples of unreacted control fibers, and are not to be construed as limiting the scope thereof. Herein parts are by weight and temperatures are in degrees centigrade where not otherwise indicated.

Example 1 Approximately four ounces of 40 denier yarn composed of continuous filaments of polypyrrolidone (formed by melt extruding and oriented by cold drawing in an approximate 3.2:] ratio) are wound on a 4-inch diameter forarninous core and wet hot tempered in an autoclave with saturated steam at about 103 C. for 1 hour. After cooling and drying the yarn is rewound under approximately the same tension onto four 1 /2 inch diameter foraminous stainless steel cores, each containing about one ounce of fiber. An additional small sample is retained as a control. The four test samples are impregnated with an aqueous solution containing 37% formaldehyde and 1.7% ammonium chloride by immersion therein for 5 minutes. The four samples are then dried for minutes at 50, 60, 70 and 80 C. respectively (one at each temperature) to fix the formaldehyde and then all are placed in an oven at 150 C. for 5 minutes to effect crosslinking. They are washed to remove residual catalyst and dried. Rating of the different samples for fibrillation as hereinabove described shows that none of the cross-linked yarns fibrillates under the test conditions, therefore all rate 0 as the fibrillation score. The control sample of the same yarn not cross-linked (but hot tempered), rates 10, that is it fibrillates badly.

The samples are further tested by conventional methods to determine tenacity, elongation at. break, modulus of elasticity in grams per denier at 1% elongation and percent shrinkage on boiling for 10 minutes in water under 0.01 gram per denier tension followed by conditioning at 65% relative humidity at 72 F.

The results for the above four lots (distinguished by the temperature of drying) and the control are shown in Table I.

Five specimens about 8 /2 inches sq. are cut from taffeta fabric woven entirely from 40 denier (15 filament) polypyrrolidone yarn. One sample serves as a control and receives no treatment, another (designated No. 1) serves to show the effect of omitting the hot tempering step of the process. The other three specimens are mounted on 8 inch sq. pin frames and the: mounted specimens are then placed in a circulating air oven maintained at 230 C. Individual specimens are removed after 2, 3, and 4 minutes of dwell time (designated Nos. 2, 3, and 4 respectively). The control specimen (No. 1) which received no hot tempering is similarly mounted on a pin frame. The latter and the three hot tempered specimens (all dry) are immersed in a 5% solution of ammonium chloride in water for 5 minutes, removed and dried in a vacuum oven at 55 C. for 15 minutes. The four impregnated specimens still on the frames are then placed in a vessel heated to C. and gaseous formaldehyde (from heating a slurry of paraformaldehyde in mineral oil) is passed over them for four minutes. The specimens are thereafter washed to remove ammonium chloride and dried. The uptake of formaldehyde is about 5%. When the uptake is higher, e.g. up to about 10%, it is sometimes found that part of it is not chemically bound but is present as solid para-formaldehyde observable microscopically. Fibers are obtained from each of these four cross-linked specimen and from. the control by ravelling the specimens so that warp fibers from the treated areas are obtained and the fibers thus obtained are tested as in Example 1 above. The results are shown in Table TAB LE II Tenacity Elonga- Modulus Shrinkage Fibrilla- Specimen gr. per on (gr. per (percent) tion denier (percent) denier) (Score) Control. 2. 7 27 7 42 10 1 1. 2 67 4 44 0 Useful products, for example, non-woven fabrics havwhich are cross-linked, thus being non-fibrillating, but not heat-set. These fabrics stretch when wet and shrink when dry, much like certain natural products such as chamois leather.

Example 3 A bolt of taffeta similar to that employed in Example 2 containing yards of material inches wide is wound on a stainless steel mandrel, 8 inches in diameter, which fits a commercial pressure dyeing machine. Without mechanical modification, such a machine does not maintain the lateral tension which is desirable; but useful results are obtained. The roll is placed in the machine and hot tempered by circulating water at 105 C. under pressure therethrough for one hour, after which the roll is removed and the fabric transferred to and dried in a hot tenter frame by infrared lamps in a single pass with a dwell time of about seconds. I

A twelve yard length of the goods is run through a dip tank containing 5 gallons of 37% aqueous formaldehyde solution containing 2% of ammonium chloride. The excess solution is removed by passing the goods through a padding machine at 80 pounds roll pressure so that the uptake of solution is about -75% by weight.

The goods are then threaded into a loop drying oven and held at 85-90 C. for 30 minutes to effect drying with fixation of the formaldehyde and cross-linking. They are washed to remove residual ammonium chloride and a piece is ravelled to provide test yarns as in Example 2. Both warp and woof are tested and both score 0 for fibrillation. As noted, because of the lack of lateral tension the woof has a tenacity of 1.1 grams per denier and modulus of 4.0 while the warp is fully beneficiated with tenacity of 2.0 grams per denier and modulus of 11. Nevertheless, both have 0% shrinkage and are non-fibrillating. Treatment at higher temperatures as in examples, provides yarns having higher values of modulus.

Example 4 This example shows the effect of variation in composition of the treating solution employed. The procedure is that of Example 1 above.

Yarn specimens on cores are saturated with aqueous formaldehyde solutions of various concentration containing ammonium chloride at several concentrations for 5 minutes and then dried at 60 C. for 15 minutes to fix the formaldehyde and cross-linked by heating for 5 minutes at 150 C. The specimens are washed and dried and tested as above. The variations in concentrations in the saturating solutions and the results of the tests are Example 5 An alternative procedure which may be employed in the process of the invention is to subject the yarn continuously to the aldehydic agent and catalyst. In this process dwell times are generally much shorter than employed in the batch-type procedures described above. Adjustment of the temperature of cross-linking or curing for a given dwell time is found to be desirable.

Yarn such as that used in Example 1 is saturated (10 seconds dwell time) with a solution containing 37% formaldehyde and 1.7% ammonium chloride and is dried for 1 minute at 61 C. It is then cured or crosslinked for 1 minute at various temperatures. The variations in properties of the yarn (after washing and drying) are compared in Table V.

TABLE V Curing Tenacity, Elonga- Fibril- Lot temp., C. gJdcn. tion, Modulus lation percent rating It will be seen that as low a temperature as 80 C. gives appreciable reduction of fibrillation while at a dwell time of 1 minute, a curring temperature of 100 C. gives substantial improvement.

Similarly, it is found that when temperatures as low as 50 C. and dwell times of up to 60 minutes are employed, satisfactory benefication of the polypyrrolidone fibers is achieved.

What is claimed is:

1. The process for 'beneficiating polypyrrolidone fibers Which consists essentially in subjecting polypyrrolidone fibers to the action of formaldehyde in the presence of an acidic methylolation catalyst under non-solubilizing conditions, and removing water to effect crosslinking.

2. The process according to claim 1, in which the fiber has previously been dimensionally stabilized by heatsetting.

3. The process according to claim 1, in which the fiber has previously been oriented and dimensionally stabilized by heat setting.

4. The process for beneficiating polypyrrolidone fibers which consists essentially in impregnating polypyrrolidone fibers with an acidic methylolation catalyst, subjecting the tabulated in Table III. 50 impregnated fiber to the action of sufficient formaldehyde TABLE III Lot NH C1 HCHO Tenacity, Percent Modulus Fibrillation cone. cone. gJden. elongation rating A 0. 7 37 2. 5 19 10 7 B 1.2 37 2. 5 l0 22 5 C 1. 7 27 2. 6 19 18 7 D 2. 5 37 2. 2 16 19 0 E 3. 0 37 1. 5 15 15 0 Longer periods of treatment with formaldehyde improve the fibrillation rating of Lots A, B and C.

The gel content of several formaldehyde-treated fiber samples is determined and the range of resulting fibrillation test ratings are tabulated as follows:

to bring about reaction with the fiber of about 0.1 to 1 0% by weight of formaldehyde, based on fiber weight, and heating the resulting methylolated fiber to a temperature in the range of about 50 to C. to effect crosslinking.

S. The process according to claim 4, in which the acidic methylolation catalyst and the formaldehyde are introduced simultaneously.

6. The process according to claim 4, in which the formaldehyde is in aqueous solution.

7. The process according to claim 4, in which the catalyst is introduced in aqueous solution to saturate the fiber, the water content of the saturated fiber is reduced to less than about 10 percent by weight, and the 9 fiber is then subjected to the action of gaseous formaldehyde.

8. The process of claim 4 which the methylolation catalyst is ammonium chloride.

9. Beneficiated polypyrrolidone fibers containing an amount of chemically bound formaldehyde effective to substantially reduce the fibrillation properties thereof produced by the process of subjecting polypyrrolidone fibers to the action of formaldehyde in the presence of an acidic methylolation catalyst under non-solubilizing conditions and removing water to effect cross-linking.

10. Beneficiated polypyrrolidone fibers containing from about 0.3 to about 5 percent by weight of chemically bound formaldehyde produced by the process of subjecting polypyrrolidone fibers to the action of formaldehyde in the presence of an acidic methylolation catalyst under non-solubilizing conditions and removing Water to eflect cross-linking.

11. Beneficiated polypyrrolidone fibers having gel content above about 80 percent and characterized by improved resistance to fibrillation under Wet abrasive c0nditions produced by the process of subjecting polypyrrolidone fibers to the action of formaldehyde in the presence of an acidic methylolation catalyst under non-solubilizing conditions and removing water to effect cross-linking.

References Cited by the Examiner UNITED STATES PATENTS 2,177,637 10/1939 Coifman 8 2,293,986 8/1942 Koch '8-l27.6 2,540,726 2/1951 Graham et :al. 8-115.5 2,734,004 2/1956 Robinson 117l39.5

OTHER REFERENCES Jenkins: Protein Fibers, Synthetic, Encyclopedia of Chemical Technology, vol. 11, Interscience Encyclopedia, New York, 1953, pp. 2084-09.

NORMAN G. TORCHIN, Primary Examiner.

I. C. CANNON, Assistant Examiner. 

1. THE PROCESS FOR BENEFICIATING POLYPYRROLIDONE FIBERS WHICH CONSISTS ESSENTIALLY IN SUBJECTING POLYPYRROLIDONE FIBERS TO THE ACTION OF FORMALDEHYDE IN THE PRESENCE OF AN ACIDIC METHYLOLATION CATALYST UNDER NON-SOLUBILIZING CONDITIONS, AND REMOVING WATER TO EFFECT CROSSLINKING. 